In book Bitcoin and Cryptocurrency Technologies: A Comprehensive Introduction (a book recommended at CS 251 Stanford course) it states that:
Mixes are businesses and expect to be paid for their services. One way for a mix to charge fees is to take a cut of each transaction that users send in. But this is problematic for anonymity, because mix transactions can no longer be in standard chunk sizes. (If users try to split and merge their slightly smaller chunks back to the original chunk size, it introduces serious and hard-to-analyze anonymity risks because of the new linkages between coins that are introduced.)
btw: definition of mixers: Mixing. Users send coins to an intermediary and get back coins that were deposited by other users. This makes it harder to trace a user’s coins on the block chain.
I could not understand the second paragraph. How it introduces serious anonymity risks? New linkages only happens inside mixers' addresses which are not related to users. What am I missing?